Previous studies have found that reward effect is stronger for more difficult to retrieve items, but whether this effect holds true for the associative memory remains unclear too. We investigated the effects and neural mechanisms of the different unitization depths and reward sets on encoding associative memory using event-related potentials, which were recorded through a Neuroscan system with a 64-channel electrode cap according to the international 10–20 system, and five electrodes were selected for analysis. Thirty healthy college students took part (...) in this study. During encoding, participants were carried out two encoding tasks, a congruity-judgment task with high unitization and a color-judgment task with low unitization, with half of the items rewarded. The test phase was conducted immediately after the encoding phase. The results for false alarm rates and Prs in relational retrieval revealed that the reward differences in the color-judgment task were greater than those in the congruity-judgment task. The ERP results further showed significant reward effects at P300 and LPP in the color-judgment task both for intact and rearranged items, and the reward effects at LPP were distributed more widely than the reward effects at P300 in the color-judgment task. These results suggest that reward provided a greater boost when retrieving associative memory of low unitized items. (shrink)